For a long time bituminous emulsions have been prepared by cold coating of granulates with the aid of bituminous emulsions. In this application, cationic emulsions have been particularly valued, both for the speed of the breaking of the emulsion on the granulates, which in the case of the bituminous mix eliminates the risk of a separation due to spontaneous draining of the binder which is still emulsified, and for the adhesiveness properties of the bituminous mix, which rapidly protects the structures produced with these materials from washing out on the first occurrence of bad weather.
The factors to guard against in the production of a bituminous material by coating a size-graded composition with a bituminous emulsion are well known to a person skilled in the art:
during the coating an excessively fast destabilization of the emulsion on contact with the granulates, which has the effect of a poor final distribution of the binder; PA1 during the conveying of the bituminous mix, an excessive sensitivity of the bituminous mix to temperature, to evaporation and to vibrations, which has the effect of making it difficult to unload and to make its placing in position or its compacting difficult or even impossible.
Among the simple remedies, the increase in the water content during the coating allows a better distribution of the binder within the granulate. The converse side is the slowness of breaking of the emulsion on coating, or even overstabilization, which results in the loss of unbroken emulsion and the outflow of dirty liquors during conveying and the risk of the bituminous mix being delivered on site in the form of "soup", and hence severe difficulties in order to remove the excess water on compacting and to obtain a large increase in cohesion. The additional risk is that with excessively long periods between the coating and the delivery on site, the bituminous mix is delivered on site solidified in the tipper, deprived of handleability when unloading, when being spread and finished and when being rolled.
The use of a soft bituminous binder very certainly improves the handleability of the bituminous mix, but at the cost of a low and unacceptable mechanical strength of the structure. In all cases the behaviour of the bituminous mix is very dependent on its water content, the working range of the water content is very narrow and, furthermore, poorly controllable.
The less open the bituminous mix manufactured, the more these features are accentuated, and improvements are still to be expected for the manufacture of bituminous mixes, especially of cold dense bituminous mixes. (Cold dense bituminous mixes are intended to mean the systems in which the granular fraction comprises a 2 mm sieve undersize of at least 25% and an 80 .mu.m sieve undersize of at least 5% of its total mass.)
Insofar as the handleability factor of the bituminous mix is concerned, an appreciable improvement has been obtained, described in French patent No. 92 10908 (publication FR 2 695 664), which consists in producing a bituminous mix with a binder of given hardness, with an emulsion containing two bitumens in the dispersed state, one hard, the other soft, in proportions such that their mixture has the desired final hardness. The handleability when being placed in position is facilitated by the soft bitumen, whereas the final quality of the structure is attained by the formation in position of a binder of desired hardness by fusion of the droplets of hard bitumen with those of soft bitumen. However, this process does not provide a solution to the control of the speed of breaking of the emulsion and hence of the physicochemical change in the granulate/emulsion system between the coater and the compacting roller.